3
$\begingroup$

UV radiation isn't visible to the human eye, so how come we can see it as a purple/violet light from a UV lamp? Is it just because the lamps aren't perfect and end up emitting some light at a higher frequency? Or do they add some purple light intentionally? Or is there some more complex mechanism going on?

$\endgroup$
1
  • $\begingroup$ If a lamp sends out only one wavelength, it would be a laser. Pretty expensive an unnecessary for most purposes. Instead a broader spectrum of wavelengths is accepted with highest intensity in the UV range. $\endgroup$
    – Steeven
    May 18, 2016 at 21:48

2 Answers 2

3
$\begingroup$

There are several kinds of "UV Light." Only kind I know of that "looks purple" is a so-called black light.

Black lights emit UV that is very close to the top-end of the visible spectrum. The designers try to minimize the visible radiation so that it won't wash out the light emitted by fluorescent substances in the field, but it's hard to filter all of the visible out.

The visible tail looks "purple" because the "red" receptors in your eye have some sensitivity at the shortest visible wavelengths. The visible leakage from a black light stimulates both "red" and "blue" receptors in your eye, and you perceive purple.

Other types of UV lamp (e.g., germicidal lamps) generally put out a lot of visible light that they don't bother to filter out because (A) it does not interfere with the application (e.g., visible light won't stop the UV from killing the germs) and (B) for safety reasons (i.e., so you know when you're looking at the light, which you shouldn't because it's dangerous.)

$\endgroup$
1
  • 1
    $\begingroup$ The 'red' cones don't really have a bump; it's just that the tail of the excitation spectrum decreases to zero a bit more slowly than the tail of the 'blue' cones, so that the perceived red/blue ratio increases as one goes to short (blue) wavelengths. $\endgroup$ May 22, 2016 at 12:08
3
$\begingroup$

The uv light often originates from excited Mercury atoms which are in the bulb. Excited Mercury atoms also produces violet and blue visible light.

$\endgroup$
1
  • $\begingroup$ If anything the effect is more noticeable with LEDs than I remember with the old tube lights (at least the ones we used in discos), but I haven't seen one the of the tubes in years. $\endgroup$ Oct 24, 2020 at 23:42

Not the answer you're looking for? Browse other questions tagged or ask your own question.